1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image used upon developing an electrostatic latent image formed by an electrophotographic process or an electrostatic recording process with a developer, and a process for producing the same.
2. Description of the Related Art
A method for visualizing image information through an electrostatic image by an electrophotographic process has been utilized in various fields of art. In the electrophotographic process, an electrostatic image is formed on a photoreceptor through charging and exposing steps, and the electrostatic latent image is developed with a developer containing a toner and then visualized through transferring and fixing steps. The developer used herein is classified into a two-component developer containing a toner and a carrier and a one-component developer using a magnetic toner or a non-magnetic toner solely. The toner is generally produced by a kneading and pulverizing method, in which a thermoplastic resin is melted and mixed with a pigment, a charge controlling agent and a releasing agent, such as wax, and after cooling, the mixture is finely pulverized and then classified. Inorganic or organic fine particles may be added to the surface of the toner particles depending on necessity in order to improve the flowability and the cleaning property thereof.
In recent years, duplicators, printers and complex machines of them with a facsimile machine by means of color electrophotographic process are significantly spread. In order to obtain moderate glossiness for reproducing a color image and to realize transparency for obtaining an excellent OHP image, it is generally difficult to use a releasing agent, such as wax. Accordingly, a large amount of an oil is applied to a fixing roll to assist releasing, which causes stickiness of a duplicated image including an OHP image and difficulty in writing on the image with a pen, and also heterogeneous glossiness often occurs.
In the ordinary monochrome (black and white) duplication, furthermore, wax, such as polyethylene, polypropylene and paraffin, which is generally used in a monochrome toner, is difficult to use because the OHP transparency is liable to be deteriorated.
Even though the transparency is sacrificed, it is difficult to suppress exposure of wax on the surface by the production process of the toner using the conventional kneading and pulverizing method. Therefore, such problems are caused upon using as a developer, as considerable deterioration in flowability and filming on a developing device and a photoreceptor. Hardware designs are in progress especially for color and/or toner reclaiming process.
Intermediate transfer system and/or toner reclaiming process of remaining toner on photoreceptor require more reliability of toner and developer materials.
As an ultimate method for removing the problems, such a production process by a polymerization method is proposed that an oily phase containing a monomer as a raw material of a resin and a colorant is dispersed in an aqueous phase and then is directly polymerized to form a toner, whereby the wax is encompassed in the interior of the toner to suppress exposure thereof on the surface.
Furthermore, as a method for intentionally controlling the shape of the toner and the surface structure thereof, production processes of a toner by an emulsion polymerization and aggregation method are proposed in JP-A-63-282752 and JP-A-6-250439. In these production processes, generally, a resin dispersion is produced by emulsion polymerization, and separately, a colorant dispersion is produced by dispersing a colorant in a solvent. The dispersions are mixed to form aggregates having a diameter corresponding to the particle diameter of the toner, which are fused and coalesced by heating to form the toner.
These production processes not only realize internal inclusion of wax, but also facilitate to form toners having small diameters and enable reproduction of an image with high resolution and high sharpness.
In the case where these production processes are employed, designing of characteristics of the resin is significantly important for realizing high image quality. In order to obtain a broad color reproduction band, not only optimization of a colorant but also formation of an image with glossiness of a certain level or higher through the melting characteristics of the resin are required. On this account, the resin is designed in such a manner that the elasticity thereof is decreased to lower the melt viscosity upon heating with a heating roll, so as to facilitate flow thereof. In order to realize such a nature, the molecular weight of the resin is necessarily decreased. However, in the case where the elasticity is decreased, the adhesiveness to the heat roll is increased, and it is difficult to release from the roll without the presence of a fixing oil even when a releasing agent, such as wax, is contained. Furthermore, hot offset at a high temperature is liable to cause a problem due to the low molecular weight, and as a result, the resulting toner is liable to have a significantly narrow usable temperature range.
Moreover, in the case where the molecular weight is decreased, the glossiness is increased to improve the image quality, but the fixed image is liable to become mechanically brittle. Therefore, image defects are liable to be formed due to folding of a medium, such as paper, and thus a problem occurs on the standpoint of durability of images.
As described in the foregoing, in order to provide a high quality image through the electrophotographic process and to maintain the stable performance of the toner under various kinds of mechanical stress, it is necessary that the species and the amounts of the pigment and the releasing agent are optimized, and the releasing agent is suppressed from exposing on the surface, and it is also important that the glossiness and the releasing property without the presence of a fixing oil are improved through optimization of the characteristics of the resin, and hot offset is suppressed.